1. Field of the Invention
The present invention relates to a method and apparatus for measuring turbidity.
2. Description of the Related Art
A turbidity meter is used for measuring the turbidity of raw and purified water in water treatment in which a transmitted light, a scattered light, a surface scattering light, and a transmitted--scattered light system are employed.
In the transmitted light system, there follows such a method that when specimen water flowing through a flow cell is irradiated with a light beam from a light source, the light transmitted through the fluid is received by a photoelectric converter, and the voltage converted therein is reconverted into turbidity. This system is fit for examining specimen water having high turbidity.
In the scattered light system, there follows such a method that when specimen water flowing through a flow cell is irradiated with a light beam from a light source, the light scattered by fine particles within the fluid in received by a photoelectric converter, and the voltage converted therein is reconverted into turbidity. This system is fit for examining specimen water having low turbidity.
In the surface scattering light system, there follows such a method that when the surface of specimen water is not irradiated via a flow cell but directly irradiated with a light beam from a light source, the light scattered by fine particles in the proximity of the surface of the specimen water is received by a photoelectric converter, and the voltage converted therein is reconverted into turbidity. This system features that the specimen water remains unaffected by the contamination of the flow cell because the flow cell and the specimen water are not brought into contact with each other in the light beam irradiation area.
In the transmitted--scattered light system, a quantity resulting from dividing scattered light intensity by transmitted light intensity is reconverted into turbidity. This system allows measurement of from low turbidity up to high turbidity.
Close control over purified water quality is increasingly exercised in order to cope with cryptospolydium and the like, and according to the tentative guiding principle announced by the Ministry of Welfare reads "Turbidity shall be maintained at 0.1 or lower at the exit of a filter reservoir of any purification plant where raw water for city water may be contaminated by cryptospolydium." As a result, an on-line turbidity meter capable of stably measuring a turbidity of 0.1 or lower becomes essential. However, it is difficult for a typical conventional turbidity meter to measure such a low turbidity because the conventional turbidity meter is designed to catch the scattering and transmittance of a light beam due to fine particles in the form of groups as the number of fine particles within an observation area considerably small. In order to make the aforementioned measurement possible, the probability of the presence of fine particles has to be increased by modifying the conventional turbidity meter in that the optical path length is increased in the transmitted light system and otherwise the light beam irradiation area is enlarged in the scattered light system. However, since the aforementioned modification results in enlarging the optical system of the turbidity meter, attaining two-digit high sensitivity in the turbidity measurement is hardly possible when size restriction is taken into consideration.
Further, membrane treatment technology is beginning to be employed in the water treatment and in order to secure the stability of treated water obtained from the membrane treatment, a turbidity meter and a fine particle counter are used for monitoring the treated water. However, the conventional turbidity meter is usable for detecting only a case where the rupture of membranes causes a considerable quantity of raw water to flow into the treated water, and is unable to detect the outflow of a small quantity of raw water when some membranes are cracked; the indicated value of turbidity at this time is almost zero like the water subjected to normal membrane treatment. It is therefore difficult to discover the abnormal condition of treated water through turbidity measurement in its early stages. On the other hand, the fine particle counter is used for measuring the number concentration of fine particles having a particle diameter greater than a nominal pore diameter of the membrane in water treatment so as to monitor an increase in fine particles when the membrane is ruptured or cracked. As the number concentration of fine particles is obtained by counting the optical pulses scattered or intercepted by the respective fine particles one by one, sensitivity is good in comparison with turbidity in a region where the probability of the presence of fine particles in the observation area is low, so that the fine particle counter is fit for detecting the abnormal condition of the membrane. However, care should be taken to observe an error in miscounting the number of fine particles when the probability of presence of fine particles in the observation area increases.
In the case of measurement in an extremely low turbidity region, that is, in a case where treated water and membrane treated water at a turbidity of 0.1 or lower, the sensitivity of the conventional turbidity meter is insufficient and it is consequently preferred to use the fine particle counter for measuring the number concentration of fine particles within treated water. However, there is a history that turbidity has been measured for many years in the field of water treatment, and there is no experience that water quality is determined on the basis of a value of the number concentration of fine particles which is measured in a purification plant or the like.